Jesse Greenstein was born in New York and educated at Harvard University, with four depression years spent in business between his master’s degree and Ph.D. studies. He worked at Yerkes Observatory, which included McDonald Observatory, from 1937 to 1948. He became the founding head of the graduate program in astronomy at the California Institute of Technology at the time of the inauguration of the 200-inch telescope and the joint operation of Palomar and Mt. Wilson Observatories, and he directed the Caltech astronomy program until 1972. A spectroscopist with interests in theory and instrumentation, he explored the interstellar medium, the colors of nebulae, abundances of the elements and isotopes, and peculiar stars. His abundance project showed that stars in globular clusters and the galactic halo have smaller abundances of heavy elements and are therefore older than the Sun. He made detailed studies of white dwarf stars, observing hundreds with the Hale Telescope and determining their masses, luminosities, temperatures, compositions, gravitational redshifts, magnetic fields, and motions. During World War II he and Louis Henyey developed a wide-field camera which had both military and astronomical applications. He collaborated in the discovery of quasars by Maarten Schmidt in 1963. Greenstein was an early supporter of radio astronomy, and instigated the founding of Caltech’s Owens Valley Radio Observatory. He was an important advisor to government agencies and held many administrative positions, including chairing the National Research Council Astronomy Survey Committee, which published the astronomical community’s recommendations for funding for astronomy and astrophysics in the 1970s. In 1995, long after retirement, he revisited the prime focus cage of the Hale telescope.